Meaning
A round black bomb with a lit fuse, drawn in cartoon style rather than as any real device. It stands for explosions, danger and something about to blow up.
In chat it usually means something dramatic or impressive, like shock news or a showstopping outfit: “She just dropped a 💣 in the group chat, they’re engaged!” It can also warn that a situation is about to explode; it is best avoided when talking about real violence.